1. Field of the Invention
The present invention relates to an optical printer head and, more particularly, to the arrangement of light emitting elements provided in the optical printer head.
2. Description of the Prior Art
In optical printer heads, a plurality of light emitting diodes may be arranged in rows in a manner that light beams from the light emitting diodes converge in a selfoc lens array to form a latent electrostatic image on a right circular cylinder-shaped photosensitive drum which has previously been electrostatically charged. Each light emitting diode 1 has an electrode 2 to which power is supplied to cause a light emitting region 3 to emit light. The direction of movement of the photosensitive drum is perpendicular to the orientation of the light emitting diodes 1 (vertical direction in FIG. 1(1)) as indicated by an arrow 4.
A brightness profile of the light emitting diode 1 taken along a line B1--B1 which is in parallel to the direction of drum movement 4 is illustrated in FIG. 1(2), and a brightness profile of the light emitting diode 1 taken along a line A1--A1 which is transverse to the direction of drum movement 4 is illustrated in FIG. 1(3). A developed dot to be formed on the photosensitive drum as a result of light emission from the light emitting diode 1 is represented by reference numeral 5 in FIG. 1(4). A light beam emitted from the light emitting region 3 of the light emitting diode 1 passes through a selfoc lens array (not shown) which is disposed between the light emitting diode 1 and the photosensitive drum and forms an image on the photosensitive drum. In FIG. 1(5), there is shown a light intensity profile of the light beam which has passed through the selfoc lens array and which is taken in a direction parallel to the direction of drum movement 4. A light intensity profile transverse to the direction of drum movement 4 is illustrated in FIG. 1(6). As light passes through the selfoc lens array, marginal regions 6, 7 of the brightness profile of the light from the light emitting region 3 of the light emitting diode 1 are significantly lowered into minor margins 8, 9.
In a conventional optical printer head of such arrangement, as FIG. 2(1) illustrates, the light emitting diodes 1 are driven to effect light emission from their light emitting regions 3. Scanning along a horizontal scanning direction or along the orientation of the rows of the light emission diodes is repeated, for example, at intervals of time period T1. As a result of the rotation of the photosensitive drum in a vertical scanning direction perpendicular to the horizontal direction and the repeated light emission from the light emission diodes, latent images of lines are formed on the photosensitive drum in the vertical scanning direction. FIG. 2(2) shows a latent image of a continuous line having a width W1 which is formed in the vertical scanning direction on the photosensitive drum by one of the light emission diodes. The line having the width W1 is thereafter transferred onto a transfer paper.
When the light emitting diodes 1 are driven at interval of time period T2, for example, in order to effect intermittent printing, as illustrated in FIG. 3(1), a line having a width W2 which is narrower than the width W1 in the case of continuous printing is printed in transfer paper as shown in FIG. 3(2), whose image is found to be sporadically broken.
When one dot only is to be printed, the light emitting diodes 1 are driven to emit light thereby to effect printing on transfer paper in manner as shown in FIG. 4(2). The printed dot is found to be out of shape when compared with the light emitting region 3 of the light emitting diodes 1. For the purpose of printing such one dot, the light emitting diodes are driven at time intervals T3 as shown in FIG. 4(1).
Reasons why continuous printing and intermittent printing results in such different print line widths W1, W2, and why one dot printing involves such deformation in print configuration will be explained. When light from each light emitting diode 1 passes through the selfoc lens array to form an image on the photosensitive drum, the light is decayed by the selfoc lens. For example, a certain selfoc lens array can reduce the light intensity to as low as about 1/5 thereof. Further, the intensity profile of the light which has just passed the selfoc lens array is such that, as already stated, the light intensity of the marginal region is significantly lowered. In addition, where the photosensitive drum is in rotation while a latent electrostatic image is being written on the drum, the light from the light emitting diode does not focus on one point and, if the duration of light emission is short, the trouble is that writing energy is insufficient. The profile of such reduced light intensity on the photosensitive drum involves the following problems at time of printing, as FIG. 5 illustrates. In the case of continuous printing, as FIG. 5(1) shows, the presence of marginal regions 8 (FIG. 1(5)) and 9 (FIG. 1(6)) in the intensity profile of light beams 10 from the light emitting diode 1, coupled with the fact that light emitting time interval Ti is short, causes overlapping of emitted light beams 10 in a similar manner as in the case of light emission being effected for a longer period of time. As a result of the repeated overlapping of the light beams, the light beams form a latent image of a continuous line 11 having a large width on the photosensitive drum which is printed accordingly on the transfer paper.
In the case of intermittent printing, as FIG. 5(2) shows, there develops a smaller degree of overlapping of emitted light beams 12 than in the case of continuous printing, with the result that a continuous line 13 of a smaller width than in the case of continuous printing is written as a latent image on the photosensitive drum.
In the case of one dot printing, as FIG. 5(3) shows, no overlapping effect occurs between an emitted light beam 14 and other adjacent light beams, and accordingly the resulting print dot 15 is diametrically smaller. After light beams from individual light emitting diodes have passed through the selfoc lens array, marginal regions 8, 9 of the intensity profile of the light beams are already lower in their intensity as earlier stated in conjunction with FIGS. 1(5) and 1(6), and therefore the energy available for exposure of the photosensitive drum is insufficient. Furthermore, since the photosensitive drum is in rotation, the light from the light emitting diodes does not focus on one point. This is another cause of the insufficiency of exposure energy.
The above mentioned problems of the prior art arrangement as explained with reference to FIGS. 1 to 6 are largely accounted for by the configuration of the light emitting region 3 of each light emitting diode. In the light emitting diode 1 shown in FIG. 1(1), the current for driving it flows at a large current density in the proximity of the electrode 2 and, as the current flows away from the electrode 2, rightward in FIG. 1(1), the density of the current is abruptly reduced so that as already mentioned, the brightness profile of the light from the light emitting diode 1 is as shown in FIG. 1(2) wherein the brightness of the marginal region 6 is considerably lower than that in the proximity of the electrode 2. Such reduced brightness of the marginal region 6 is attributable to the configuration of the light emitting diode 1. This phenomenon is particularly apparent when light from the light emitting region 3 of the light emitting diode 1 has passed through the selfoc lens array. That is, as can be seen from FIG. 1(4), the width of the emitted light beam 5 becomes smaller and, especially in the case of one dot printing, each print dot is extremely small in its diametrical size.
FIG. 6 shows the relationship between input energy of light emitting diodes as one part and breadth of distribution of light beam brightness and print line width as the other part. The light emitting diodes 1 are arranged in a dot density of 300 dots/inch. Line l1 represents the breadth of the brightness distribution of a light emitting region 3; line l2 represents line width W in the case of continuous printing; and line l3 represents dot width W3 in the case of one dot printing.
It can be seen that in the case of continuous printing a large line width can be obtained when the input energy is low, as line l2 indicates, whereas in the case of one dot printing the diameter of each dot print is not so large as that in the case of continuous printing. In order to obtain a larger diameter of a dot, which is comparable to the continuous print line, a large input energy has to be applied as shown in FIG. 6. A comparison between line l1 indicative of the breadth of the brightness distribution and line l2 indicative of the width of continuous print line shows that there is a difference of 4 to 5 times, that is, the latter is 4 to 5 times larger than the former. The reason for this is that the width of emitted light beam is increased before the light beam from each light emitting diode reaches the surface of the photosensitive drum, and that overlapping of emitted light beams in continuous printing results in increased print line width.
In summary, printing by using the prior art arrangement of light emitting diodes 1 involves that problem that the resulting print lines and dots differ in width and diameter according to the cyclic period of printing, such as continuous printing, intermittent printing, or one dot printing. In the case of one dot printing in particular, no dot diameter W3 can be obtained at the required value. Therefore, with the prior art arrangement, the problem of print quality degradation is unavoidable.